Allow to call original `errorHandler` when custom one defined

[ArmorDarks]

What problem does this feature solve?

When custom Vue.config.errorHandler defined by the user, it will stop firing original logError .

The problem is that there is no straight way to recreate logError behavior in custom Vue.config.errorHandler without requiring some Vue's internals.

This issue is partially related to raven-js#1416, which defines custom errorHandler... which prevents Vue from login errors to the console.

What does the proposed API look like?

I think Vue should by default expose original errorHandler under Vue.config.errorHandler. In other words, Vue.config.errorHandler shouldn't be undefined by default.

If the user would like to override errorHandler, he will reassign it. Otherwise, it would be possible to save original Vue.config.errorHandler into some variable and call it within custom errorHandler.

[posva]

You should be able to recreate it easily:

function logError (err, vm, info) {
  if (process.env.NODE_ENV !== 'production') {
    warn(`Error in ${info}: "${err.toString()}"`, vm)
  }
  console.error(err)
}

Apart from that, this looks reasonable to me

[ArmorDarks]

warn content isn't easily re-creatable (that's what I meant when said "there is no straight way to recreate logError behavior", I had to be more specific), and it's importable only from Vue sources, that's not an option in most cases.

[b-ryan]

I just ran into this error too. It looks like the warn function is actually available via Vue.util.warn. So the custom error handler can be:

  Vue.config.errorHandler = (err, vm, info) => {
    if (process.env.NODE_ENV !== 'production') {
      Vue.util.warn(`Error in ${info}: "${err.toString()}"`, vm)
    }
    console.error(err)
  }

I am using Vue 2.6.2

[loomchild]

Since the request looks reasonable and it simplifies configuration of external loggers, I am wondering if anyone is working on a PR for this? If not I'd be happy to try to propose something (basically exposing original errorHandler under Vue.config.errorHandler).